The present invention relates to improvements in the art of preparing moldable reinforced polyesters characterized by being tack free, and by little or no tendency towards shrinkage during the curing thereof.
For many years polyester resins based on an unsaturated polyester and an unsaturated monomer, such as styrene, have been used in the preparation of molding compositions. When formulated in a known manner with appropriate adjuvants such as fillers, pigments, curing agents, reinforcing agents, and the like, such polyester-based molding compositions may be used in conventional molding processes such as sheet molding or bulk molding to prepare a wide variety of materials or articles which are generally characterized by high strength, light weight and excellent chemical resistance. However, articles fabricated from such molding compositions by conventional molding techniques commonly exhibit poor dimensional stability, due in a large part to shrinkage during the molding process. Thus, considerable difficulties are encountered in the production of molded articles where close size and shape tolerance limits are required since in many instances an inordinate amount of labor is required after removal of a part from the mold to shape it to the exact finish, size or shape required for a given application.
Considerable effort has been expended in recent years, in the development of dimensionally stable, low profile polyester molding compositions. It is now well known to those skilled in the art that with the addition of thermoplastic polymers to polyester systems, such as polyester-styrene based resins, there may be prepared compositions which, when formulated with the appropriate adjuvants to form molding compounds, display minimal shrinkage or expansion during the molding process and may be formed with smooth surfaces (low profile). The thermoplastic polymers which have been used for this purpose include, for example, such materials as polyvinyl acetate, cellulose acetate, cellulose butyrate, polymethyl methacrylate, polystyrene, polyethylene, polyvinyl chloride and saturated polyesters.
With the advent of low-profile unsaturated polyester systems, molding techniques have been increasingly employed in the fabrication of automotive parts such as hood scoops, finder extensions and a wide variety of other parts which require both a smooth surface and close adherence to the size and shape of a precision machined mold.
Accordingly, it is an object of the present invention to provide polyester based molding compositions which exhibit a high degree of dimensional stability. It is further object to provide low-profile additive compositions which may be added to polyester based molding compositions to impart a high degree of dimensional stability thereto. It is an additional object to provide articles of manufacture molded from novel polymeric compositions and closely reproducible in size and shape. Other objects and advantages of this invention will be apparent from the following description of the present invention.